1. Field of the Invention
The present invention relates to a lithium secondary battery and, more particularly, to a lithium secondary battery which exhibits excellent battery characteristics by improving the constitution of a negative electrode. The present invention also relates to a method of manufacturing a negative electrode carbonaceous material suitable for a lithium secondary battery.
2. Description of the Related Art
Recently, nonaqueous electrolyte batteries using lithium as their negative electrode active materials have attracted attention as high-energy-density batteries. As an example, primary batteries using, e.g., manganese dioxide (MnO.sub.2), carbon fluoride [(CF.sub.2)n], and thionyl chloride (SOCl.sub.2), as their positive electrode materials, are widely used as power sources of calculators and watches and backup batteries of memories.
In addition, with recent reduction in size and weight of various electronic devices, such as VTRs and communication devices, a demand has increasingly arisen for high-energy-density secondary batteries as power sources of these devices. To meet this demand, active researches have been made on lithium secondary batteries using lithium as a negative electrode material.
These researches have been made on a lithium secondary battery comprising a negative electrode consisting of lithium; a nonaqueous electrolyte, in which a lithium salt, such as LiClO.sub.4, LiBF.sub.4, or LiAsF.sub.6, is dissolved in a nonaqueous solvent, such as propylene carbonate (PC), 1,2-dimethoxyethane (DME), .gamma.-butyrolactone (.gamma.-BL), or tetrahydrofuran (THF), or a lithium-ion-conductive solid electrolytic salt; and a positive electrode containing an active material mainly consisting of a compound which topochemically reacts with lithium, such as TiS.sub.2, MoS.sub.2, V.sub.2 O.sub.5, V.sub.6 O.sub.13, and MnO.sub.2.
No lithium secondary battery with the above arrangement, however, has been put into practical use. This is so mainly because the charge-discharge efficiency is low and the number of times by which charge and discharge are possible is small (i.e., the cycle life is short). It is considered that the major cause for this is degradation in lithium due to the reaction between lithium of the negative electrode and the nonaqueous electrolyte. That is, the surface of lithium which is dissolved as lithium ions in the nonaqueous electrolyte during discharge is partially inactivated by reacting with the nonaqueous solvent contained in the electrolyte when it precipitates from the nonaqueous electrolyte during charge. As a result, when charge and discharge are repeatedly performed, lithium precipitates into dendrites or globules or leaves a collector of the negative electrode.
For these reasons, lithium secondary batteries having negative electrodes containing carbonaceous materials which absorb and desorb lithium ions, such as coke, a resin sintered product, a carbon fiber, and pyrolytic carbon, have been proposed. A lithium secondary battery having a negative electrode of this type can reduce degradation in negative electrode characteristics by suppressing the reaction between lithium and the nonaqueous electrolyte and hence the precipitation of dendrites.
It is considered that in the negative electrode containing the above carbonaceous material, absorption and desorption of lithium ions occur in a portion of a structure (graphite structure) in which hexagonal-net-plane layers consisting primarily of carbon atoms are stacked, particularly in portions between these hexagonal-net-plane layers, thereby causing charge and discharge. It is, therefore, required to use a carbonaceous material, in which a graphite structure is developed to some extent, as the negative electrode of a lithium secondary battery. However, the negative electrode containing the carbonaceous material obtained by powdering giant crystals that are highly graphitized decomposes the nonaqueous electrolyte, decreasing the capacity and the charge-discharge efficiency of a battery. Especially when a lithium secondary battery having the above negative electrode is operated at a high current density, the capacity, and the voltage during charge-discharge of the battery decrease significantly because of the diffusion limitation caused by the slow diffusion rate of lithium ions. In addition, the crystal structure or the fine structure of the carbonaceous material collapses as the charge-discharge cycle progresses. This impairs the ability of the material to absorb and desorb lithium ions, resulting in a short cycle life.
Furthermore, like the negative electrode containing the carbonaceous material obtained by powdering giant crystals such as natural graphite, a negative electrode containing a fine powder of carbon fibers which are highly graphitized decomposes the nonaqueous electrolyte, with the result that the performance as the negative electrode is largely degraded.
A carbonaceous material consisting of coke or carbon fibers with a low graphitization degree, on the other hand, can suppress decomposition of the solvent to some extent. A negative electrode containing a carbonaceous material of this type, however, has problems of a low capacity, a low charge-discharge efficiency, a high overvoltage during charge and discharge, a low flatness of the discharge voltage of a battery, and a short cycle life.
Published Unexamined Japanese Patent Application Nos. 62-268058, 2-82466, 4-61747, 4-115458, 4-184862, and 4-190557 disclose various carbonaceous materials which obtain optimal parameters of the graphite structure by controlling the degree of graphitization. However, negative electrodes containing these carbonaceous materials do not necessarily have sufficiently good characteristics.